Field of the Invention
The presently disclosed invention relates to fossil fuel power generation facilities and, more particularly, systems for adapting such facilities for removal and capture of carbon dioxide from combustion exhaust gases.
Discussion of the Prior Art
Various commercial systems and process for combusting fossil fuels to generate electrical power have been in use for many years. One difficulty with the use of such systems has been that they emit quantities of carbon dioxide—a greenhouse gas. It is believed that greenhouse gases such as carbon dioxide cause a deleterious effect when released into the atmosphere in quantity. Accordingly, fossil fuel power plants have emphasized systems and methods having lower emissions of greenhouse gases.
One system for more efficient combustion of fossil fuel and consequently lower carbon dioxide emissions employs technology known as pressurized fluidized bed combustion. In that system, fuel such as coal is introduced into a pressurized vessel and combusted while a stream of air is forced through the fuel. This has been found to result in more complete combustion of the coal and lower emissions of carbon dioxide in comparison to some other systems and processes.
It has been observed that a process for removing and capturing carbon dioxide from the exhaust emissions of the pressurized fluidized bed combustion could further reduce carbon dioxide emissions, provided the process was compatible with the fluidized bed combustion technology. One process for removing and capturing carbon dioxide from a gas stream is known as the Benfield process. In the Benfield process, carbon dioxide and other gaseous components are absorbed in a pressurized aqueous solution of potassium carbonate. The Benfield process has been found to be effective when used in connection with pressurized fluidized bed systems, provided the operating conditions for the Benfield process are met. In particular, the maximum operating temperature, the concentrations of sulfur dioxide and nitrous oxides must be satisfied. Because the temperature, sulfur dioxide and nitrous oxide in exhaust gases from the pressurized fluidized bed combustion process are high relative to those requirements. Accordingly, an interface between the pressurized fluidized bed combustion process and the Benfield process is required.
One interface for using the Benfield process in combination with a pressurized fluidized bed combustion process is shown and described in U.S. Pat. No. 8,752,384. In that system, exhaust gas from the pressurized fluidized bed combustion vessel is provided to a heat recovery steam generator. The heat recovery steam generator uses a portion of the thermal energy from the exhaust gas to convert feed water to steam. The steam is then used to power a steam turbine generator and electricity from the steam turbine generator is used to power an electric motor that drives an air compressor. The air compressor pressurizes air that is fed to the pressurized fluidized bed combustion vessel.
Exhaust gas that leaves the heat recovery steam generator is conditioned by the removal of particulates and sulfur dioxide and then provided to the Benfield processing unit for removal and capture of carbon dioxide. During startup periods, the conditioned exhaust gas (also known as flue gas) does not meet the temperature requirements for the Benfield process so the flue gas is diverted to bypass the Benfield processing unit.
To make the system more efficient, the air compressor that pressurizes air to the pressurized fluidized bed combustion vessel is powered by a second device—a gas expander. The gas expander coverts energy in the flue gas to mechanical power in a shaft that is coupled to the air compressor.
A difficulty with such systems is that the expansion of the flue gas in the gas expander causes a drop in the temperature of the flue gas. In some cases, this can cause icing in the gas expander or can cause the flue gas to form acidic condensation in the air emission stack. This difficulty cannot be avoided by maintaining a generally higher temperature for the flue gas because such higher flue gas temperatures are incompatible with the Benfield process for removing carbon dioxide.
Accordingly, there was a need in the prior art for a power generation system wherein a pressurized fluidized bed combustion unit that employs Benfield technology to remove of carbon dioxide from exhaust gases also maintains sufficiently high temperatures in the flue gas to avoid difficulties associated with low temperature conditions in the gas expander and in the air discharge stack.